Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION): This condition is characterized by sudden loss of vision, often accompanied by pain on eye movement, and is more common in older adults. The presence of a cataract in the right eye and the patient's age support this diagnosis, although the cataract itself does not directly cause the acute symptoms. The normal pupillary reaction and absence of a relative afferent pupillary defect (RAPD) might seem to contradict this, but in some cases of NAION, these findings can be initially normal.

Acute Angle-Closure Glaucoma: Although the pupillary reaction is normal, and there's no mention of corneal edema or a mid-dilated pupil, acute angle-closure glaucoma can present with severe eye pain, decreased vision, and sometimes with a relatively normal-appearing pupil if caught early or if it's an atypical presentation. The presence of cataract and the patient's age could be risk factors.
Central Retinal Artery Occlusion (CRAO): This condition presents with sudden, painless loss of vision, but given the painful eye movements, it might seem less likely. However, the severe diminution of vision, especially with counting fingers only in the right eye, warrants consideration. The normal fundus examination does not rule out CRAO, as it might take time for retinal changes to become apparent.
Optic Neuritis: Typically presents with painful loss of vision, more common in younger populations but can occur at any age. The absence of RAPD and normal pupillary reactions might make this less likely, but it remains a consideration given the painful eye movements and vision loss.

Giant Cell Arteritis (GCA): This is a medical emergency that can cause sudden loss of vision, especially in individuals over 50. Although the question stem does not provide classic symptoms like jaw claudication or scalp tenderness, GCA can sometimes present atypically, and missing it could result in irreversible vision loss. The presence of pain on eye movement increases the suspicion for GCA.
Posterior Scleritis: Can cause severe eye pain, especially with movements, and decreased vision. It might not be as common, but given the painful eye movements, it's a diagnosis that should not be missed.

Ocular Migraine: Although more common in younger populations, ocular migraines can occur at any age and might present with transient visual disturbances, including loss of vision and pain. However, the severity and permanence of vision loss in this case make it less likely.
Intraocular Tumors (e.g., Melanoma): These can cause gradual loss of vision and might be associated with pain, although this is less common. Given the acute presentation, this is less likely but remains a consideration in the differential diagnosis for unexplained vision loss.